Apparatus of this type consists, for instance, of a hydraulic crane comprising a base column to which there is articulated a first arm to whose other end a telescopic second arm is articulated. It is important to have at all times information concerning the position of the point of application of the load at the end of the telescopic arm relative to two horizontal planes containing the pivot axis of the first arm on the base column and the pivot axis of the telescopic arm on the first arm respectively.
While it is necessary to know the direction of displacement of the first and second jack rods which make it possible to vary the angle formed between the base column and the first arm and the angle formed between the first arm and the second arm respectively, it is also important to know the effects resulting from these displacements.
As a matter of face, if the point of application of the load is located above the two planes and the maximum force tolerated is reached by emergence of the telescopic second arm, the return of the rods of the first and second jacks so as to permit a lowering of the load will correspond to an increase in the force, generating unacceptable pressures in these jacks. It is therefore advisable to prevent these return movements of the rods. On the other hand, if the emergence of the rods of the first and second jacks is started from this position, a decrease in the pressure takes place. It is therefore desirable that this movement be possible since it has no dangerous implication.
If a point of application of the load at the end of the telescopic arm is located below the two planes and if a limit value is reached by emergence of the telescopic arm, the emergence of the first and second rods will correspond to an increase of the pressure in the jacks which, if it reaches the limit of a permissible value, must be prevented.
To the extent that the point of application of the load is located between the planes, the emergence of the rod of the first jack must be possible since it corresponds to a decrease in the force, but the emergence of the rod of the second jack, corresponding to an increase of the torque, must be possible only if the pressure in the first and second jacks does not exceed the permitted value.